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| ricko_uk:
Hi, can all types of ferrite material be magnetised? For example, can ferrite rods used for antennas or those used for EMI suppression on cables or those used in transformers be permanently magnetised? What is the property (name) that describes the ability to maintain in time the magnetisation applied? Thank you :) |
| Wolfram:
The property is called magnetic remanence. Ferrite grades used for transformers and inductors are actually optimized to minimize hysteresis loss, which means that they also have very little remanence if I'm not mistaken. Note that the field strengths needed to make decent permanent magnets are extremely high, in the order of tens to hundreds of kiloamp-turns if done with an electromagnet. |
| ricko_uk:
Thank you Wolfram, comparing for example two magnets, one a Ferrite magnet (low magnetic remanence) and one a Neodinium one (high magnetic remanence) which are then both magnetised with the same current (OR with the same final strength)... If they are then left fully isolated from each other and external interferences, in time, does the ferrite magnet become demagnetised faster and sooner than the Neodenium one? Or not? Thank you |
| duak:
Ricko, ferrite materials have a range of magnetic remanence as well as other magnetic properties. Some have medium to high remanence and are used to magnets for loudspeakers and magnetrons. Others have low remanence and are used to make cores for switching power supply inductors. The way I think about ferro-magnetism is that it's about how the iron atoms in a core or magnet line up. Because of the way the electron orbitals in an iron atom (and nickel, cobalt and a few others) line up, the atoms have a natural north and soutn pole. In a piece of pure iron the atoms are pointing in differing direction and the overall magnetic field is pretty much zero. If you apply an external magnetizing force, like a current in a coil or another magnet you can get the iron atoms to line up and induce a magnetic field in the piece. When the external field is removed, most of the iron atoms snap back to a random orientation and the remanent field almost disappears, but not quite. Some iron atoms still line up and so there is now a field. If the iron is alloyed with other elements like carbon, some iron atoms are more likely to be locked into the magnetized orientation and so have greater remanence. This is why steel tools tend to get magnetized and bits and bobs get stuck to them Some elements are better than others at locking the iron atoms into a magnetized orientation than others. Aluminum-Nickel-Cobalt (ALNICO), Neodymium-Boron and Samarium-Cobalt are particularly effective. ALNICO has a problem in that it has a high remanence only when the magnetic field can be maintained at a high level after the magnetizing force is removed, ie., in a magnetic circuit. That's why ALNICO magnets had keepers between their poles or were magnetized in place like in loudspeakers. The latter two types of magnets are far better at retaining fields when taken out of a magnetic circuit. Ferrites use other elements to give the appropriate properties. Ferrite magnets aren't as strong as they have only medium remanence but they're a lot cheaper as they use more common alloying elements. Neodymium, Samarium, Nickel and Cobalt are not so common and are more expensive. Temperature is the enemy of all magnets. Higher temperatures tend to cause the iron atoms to randomize their orientations even if they are locked in place. Supermagnets are generally better in this respect than ferrite magnets. |
| Benta:
The "technical marketing" names are: "Soft Ferrite" = low remanence and coercivity for inductors "Hard Ferrite" = high ditto for permanent magnets The names are misleading, as they suggest mechanical properties. Nevertheless, these are common terms and will help you when searching. |
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